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| 1 | +// |
| 2 | +// Copyright (c) 2026 Michael Vandeberg |
| 3 | +// |
| 4 | +// Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 5 | +// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
| 6 | +// |
| 7 | +// Official repository: https://github.com/cppalliance/corosio |
| 8 | +// |
| 9 | + |
| 10 | +#ifndef BOOST_COROSIO_DETAIL_CONTINUATION_OP_HPP |
| 11 | +#define BOOST_COROSIO_DETAIL_CONTINUATION_OP_HPP |
| 12 | + |
| 13 | +#include <boost/corosio/detail/scheduler_op.hpp> |
| 14 | +#include <boost/capy/continuation.hpp> |
| 15 | + |
| 16 | +#include <atomic> |
| 17 | +#include <cstdint> |
| 18 | +#include <cstring> |
| 19 | + |
| 20 | +namespace boost::corosio::detail { |
| 21 | + |
| 22 | +/* Scheduler operation that resumes a capy::continuation. |
| 23 | +
|
| 24 | + Embeds a continuation alongside a scheduler_op so the |
| 25 | + scheduler can queue it in the same FIFO as I/O completions |
| 26 | + without a heap allocation. The continuation lives in the |
| 27 | + caller's coroutine frame (awaitable or op struct); this |
| 28 | + wrapper gives it a scheduler_op identity. |
| 29 | +
|
| 30 | + io_context::executor_type::post(continuation&) uses |
| 31 | + try_from_continuation() to recover the enclosing |
| 32 | + continuation_op via a magic tag. The tag is read through |
| 33 | + memcpy (not through a continuation_op*) so that UBSan |
| 34 | + does not flag the speculative pointer arithmetic when the |
| 35 | + continuation is not actually inside a continuation_op. |
| 36 | +*/ |
| 37 | +struct continuation_op final : scheduler_op |
| 38 | +{ |
| 39 | + static constexpr std::uint32_t magic_ = 0xC0710Au; |
| 40 | + |
| 41 | + std::uint32_t tag_ = magic_; |
| 42 | + capy::continuation cont; |
| 43 | + |
| 44 | + continuation_op() noexcept : scheduler_op(&do_complete) {} |
| 45 | + |
| 46 | + // Reactor backends (epoll, select, kqueue) dispatch through |
| 47 | + // virtual operator()(). IOCP dispatches through func_ which |
| 48 | + // routes to do_complete below. |
| 49 | + void operator()() override |
| 50 | + { |
| 51 | + std::atomic_thread_fence(std::memory_order_acquire); |
| 52 | + cont.h.resume(); |
| 53 | + } |
| 54 | + |
| 55 | + void destroy() override |
| 56 | + { |
| 57 | + if (cont.h) |
| 58 | + cont.h.destroy(); |
| 59 | + } |
| 60 | + |
| 61 | +private: |
| 62 | + // IOCP completion entry point. owner == nullptr means destroy. |
| 63 | + static void do_complete( |
| 64 | + void* owner, |
| 65 | + scheduler_op* base, |
| 66 | + std::uint32_t, |
| 67 | + std::uint32_t) |
| 68 | + { |
| 69 | + auto* self = static_cast<continuation_op*>(base); |
| 70 | + if (!owner) |
| 71 | + { |
| 72 | + if (self->cont.h) |
| 73 | + self->cont.h.destroy(); |
| 74 | + return; |
| 75 | + } |
| 76 | + std::atomic_thread_fence(std::memory_order_acquire); |
| 77 | + self->cont.h.resume(); |
| 78 | + } |
| 79 | + |
| 80 | +public: |
| 81 | + |
| 82 | + // Recover the enclosing continuation_op from its cont member. |
| 83 | + // Returns nullptr if the continuation is not tagged (bare |
| 84 | + // capy::continuation from capy internals like run_async). |
| 85 | + static continuation_op* try_from_continuation( |
| 86 | + capy::continuation& c) noexcept |
| 87 | + { |
| 88 | + // offsetof on non-standard-layout is conditionally-supported; |
| 89 | + // suppress the warning — all targeted compilers handle this |
| 90 | + // correctly and the self-relative arithmetic is move-safe. |
| 91 | +#if defined(__GNUC__) || defined(__clang__) |
| 92 | +#pragma GCC diagnostic push |
| 93 | +#pragma GCC diagnostic ignored "-Winvalid-offsetof" |
| 94 | +#endif |
| 95 | + constexpr auto cont_off = offsetof(continuation_op, cont); |
| 96 | + constexpr auto tag_off = offsetof(continuation_op, tag_); |
| 97 | +#if defined(__GNUC__) || defined(__clang__) |
| 98 | +#pragma GCC diagnostic pop |
| 99 | +#endif |
| 100 | + // Read the tag through memcpy from a char*, not through a |
| 101 | + // continuation_op*. This avoids UBSan's vptr check when |
| 102 | + // the continuation is not actually inside a continuation_op. |
| 103 | + auto* base = reinterpret_cast<char*>(&c) - cont_off; |
| 104 | + std::uint32_t tag; |
| 105 | + std::memcpy(&tag, base + tag_off, sizeof(tag)); |
| 106 | + if (tag != magic_) |
| 107 | + return nullptr; |
| 108 | + return reinterpret_cast<continuation_op*>(base); |
| 109 | + } |
| 110 | +}; |
| 111 | + |
| 112 | +} // namespace boost::corosio::detail |
| 113 | + |
| 114 | +#endif |
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